Rotary speed sensors of the type that is of interest here are used in vehicle technology to obtain one of the most important input variables for the electronic control of a vehicle. Thus, the conventional antilock brake systems (ABS), antislip controls (ARS) and vehicle stability control systems (FSR, ASMS) are forced to rely on permanent measurement and evaluation of the rotary speed response of the individual vehicle wheels. The rotary speed signals of each individual vehicle wheel are recorded and transmitted via a sensor line to an electronic control unit, for the purpose of being evaluated.
Such an active rotary speed sensor is known from International Published PCT Application No. 95 17680. The fixed part of the rotary speed sensor includes a magnetoresistive sensor element having a permanent magnet that is used as booster magnet, and besides that has electronic circuit components for signal conditioning. An active rotary speed sensor requires a current supply. The output signal of the active rotary speed sensor is a binary current signal, which is composed of impressed currents of different amplitudes. The rotary speed information is located in the frequency or in the change between the two current levels. Known rotary speed sensors of this type give rise to a square wave signal, whose frequency reproduces the measured rotary speed.
For the further processing of the information supplied by the active rotary speed sensor, a microcontroller is usually inserted, on whose input side the circuit for the signal conditioning is preconnected. The relatively cost-effective, active rotary speed sensors that are obtainable in the passenger car field are not suitable For the recording of the rotary speed in the field of commercial vehicles. These rotary speed sensors could be damaged if they come into connection with the onboard voltage of 24 Volt of the commercial vehicle, since this type of rotary speed sensor is designed only for maximum voltages of 12 Volt of the passenger vehicle electrical system.